Focussed microwave irradiation equipment is described which facilitates microdissection of rat brain nuclei and the assay of the concentrations of endogenous materials which undergo rapid metabolism during post mortem processes. We are now developing new microwave devices to inactivate enzymes in brain and other tissues of large animals and in free-moving animals. This equipment will offer an unique possibility of measuring the effect of drugs on the steady state and turnover rate of neuro-transmitters or metabolites.